Method of refining pig iron



United States Patent 11 Claims. (or. 7560) It is known to bring a liquidbath in a container of preferably round cross section into asubstantially circular swinging movement by external means and it hasbeen proposed to refine a pig iron melt, brought into a swinging motionin this manner, to steel by blowing a gaseous oxygen carrier onto orthrough the melt while applying the conditions of the correspondingsimple refining methods.

Special characteristics of such methods are contained in oldersuggestions by the applicants.

According to one suggestion the swinging motion of the bath, e.g., a pigiron bath in a container for the metallurgical treatment, especially forthe refining to steel, is brought about by applying to the containerwhich is rotatably journalled around a horizontal axis passing throughor near its centre of gravity, at its ItOP or bottom end a reciprocatingmovement at a constant frequency but at a ditferent amplitude. Afrequency is chosen which is in resonance with the rotating swingingmovement of the bath for attaining a rotating swinging movement of thesurface of the bath. For this purpose it is advantageous in a container(converter) of a cross-sectional shape other than round, suitably anelliptical cross section to arrange the initiating movement within theangle section between the longest and shortest diameters of thecontainer section. In order to achieve an irregular wave motion of thesurface of the bath the frequency of the initiating movement can beadjusted within a range of the order of 2 to 4 times as high as the onewith which the circulating swinging movement is in resonance. Accordingto another proposal the movement of the converter container can bebrought about by means of a rotating disc the axis of rotation of whichis perpendicular to the direction of movement of the converter, therebeing on the rotating disc a spindle rotatable around its axis arrangedin horizontal position so that the axis of the spindle crosses the axisof rotation of the rotating disc at right angles, a nut being providedon said spindle axially displaceable through rotation of the spindle,and a lever rotatably connected at one end to said nut the other endbeing coupled to the converter so as to transform the rotary movement ofthe spindle nut into a reciprocating movement of the converter.

Further it has been stated that by inserting a spring element, e.g. ascrew spring or a magnetic coil with spring action or several suchelastic means, it is possible to bring about the natural vibration ofthe bath promoting the intermixing of metal and slag at a smalleramplitude than by non-springy-connection between propulsion means andconverter.

Finally there has been suggested a method of intermixing liquid reactionbaths, especially for the refining reaction in the production of steelfrom pig iron, in which the surface of the bath is brought into arotating movement Within a range of revolutions giving rise to thenatural vibration of the bath which causes the intermixing of theconstituents of the surface of the bath, particularly slag and bath,said rotating movement being caused by blowing gas onto the surface sothat its impinging section describes acircular movement on the surfaceof the bath. This method is particularly suitbale for such reactions theconcentration adjustment of which between the reaction phases must be inthe neighborhood of equilibrium.

Trials carried out for the refining of pig iron to steel by blowinggaseous oxygen carriers onto a molten bath brought into a rotatingswinging movement in a melting container of circular cross section bymeans of eccentrical movement of the container gave the surprisingresult which led to a working procedure different from the procedure ofany previously known method.

When refining with oxygen blow from above it is necessary to keep theslag in liquid state during the course of tht melting in order to arriveat a somewhat satisfactory concentration adjustment between metal andslag. This is especially valid for the dephosphorization reactions inthe refining of pig iron rich in phosphorus. In a bottom blowingconverter the formation of a liquid slag after the finish of thedecarburization is necessary for this purpose. In the oxygen-converterprocess the dephosphorization must be carried out during thedecarburization period. This necessitates maintaining a liquid slagduring the decarburization. Further in these methods one or twoexchanges of the slag are necessary to arrive at the demanded lowphosphorus content at the desired decarburization point.

When refining pig iron to steel by blowing gaseous oxygen carriers fromabove the completely new knowledge was obtained that with such amovement of the bath at which natural vibrations occur which cause anintermixing of the surface layers of the bath the slag forming refiningreaction and particularly the dephosphorization by means of and belowsolid slags proceedsnot only equally well but better than with the useof liquid slags. According to this the melting process has to beadjusted so that the slag from the beginning is below its meltingtemperature, the liquidus point, i.e., that it is solid or predominantlysolid, but towards the end when that carbon content is reached, at whichit is desired to finish the refining process the slag should beliquified.

It is particularly advantageous for the attaining of such solid orsubstantially solid slags from the start to keep the temperature of themelt itself as low as possible and first towards the end to permit it torise to the desired value for tapping. This is carried out in such a Waythat suitable cooling agents are added from the start and/ or during therefining. If large additions of cooling agents bring about a commercialgain, a further advantage of a low temperature of the melt andparticularly of the maintenance of a solid or substantially solid slaglies in a low attack on the lining.

The temperature of the metal melt is thereby permitted to arrive at itsliquidus line and periodically even to pass a little below. Towards theend of the refining it is desirable that the temperature is high abovethat line. The slag temperature has thus a lower limit.

A further control lies in the adjustment of the slag composition.

When refining especially pig iron rich in phosphorus in a refiningcontainer with basic lining it is advantageous from the beginning and/orduring the refining to add so much lime to the slag components formedduring the refining that the total slag is supersaturated with CaO.Since the melting temperatures of the slag-polynary systems FeO+MnOCaO-PO or FeO+MnOCaO- SiO are essentially known it is easily possible for aman skilled in the art to choose the corresponding addition of a limecarrier.

Instead of or together with 0210 also other agents, such as MgO, whichincrease the melting point may be added.

When refining pig iron low in phosphorus but high in silicon in a basicconverter the solid slags according to aassnse the invention also can beobtained when the slag is supersaturated with 2CaO SiO a very highmelting compound. The addition of CaO carriers should then be adjustedso that this compound can be preferentially formed. A too low but also atoo high lime addition decreases the melting point and could lead toliquid slags. Also in this case carriers of other basic material thanCaO can be chosen which form high melting compounds with SiO Even whenrefining pig iron especially low in P and S in converters with acidlinings it is advantageous to work according to the invention. Then theSiO content of the slag must be so high that the slag becomessupersaturated with SiO This depends in the first place upon therelative Si content of the pig iron in relation to the contents of otherslag forming accessory elements in the pig iron.

Finally, towards the end of the refining action, however, these slagshave to be liquified. Insofar as the rising melting temperature, thelower carbon contents and the increasing content of ferrous .oxide arenot sufiicient for such a liquification, fluxing slag components canthen be added. For basic slags such additions are fluorspar or SiO orTiO or A1 and others for acid slags bases such as C210 and MgO.

The process is completed in one step, i.e. without intermediate slagtapping. Naturally it i possible to make a break for sampling ortemperature measuring or towards the end to afterrefine for correction.

Example 1 Thomas pig iron was refined to steel in an oscillating ladleof 0.8 m. interior volume. For the refining there was used 96% oxygengas, as cooling agent iron ore concentrate with 67% Fe and as slagformer burnt lime of the grain fraction 47 mm. The ox gen gas was blownthrough a water cooled tuyere of mm. inner diameter.

Charge: 1300 kgs. Thomas pig iron. Additions: 60 kgs. ore concentrate.

200 kgs. burnt lime. 87 Nm. oxygen gas.

Yield:

1122 kgs. steel 350 kgs. slag The course of the refining is clear fromthe following table giving the analyses of the metal and slag samplestaken out.

Period 0 1 2 3 Ore concentrate, kg Lime, kg 150 40 10 Oxygen, Nm. 40 3710 Blowing time, min 8. 0 7. 5 2. 5 Samples: Temperature of bath, C.1,120 1, 3t 0 1, 610 1, 555 Metal, percent' 51. 4 4G. 3 2. 8O 2. 6. 5T5. S5 17. 6 13. 8 12. 3 20. 6

Lime in the first period was added before the beginning of the blowingand after about 6 minutes ore concentrate wa added continuously. Morelime was added before the start of the second blowing period whereascooling agent was continuously added during the first two minutes of thesecond blowing period. The slag was molten first towards the end of thesecond blowing period.

Example 2 In this case a low phosphorus hot metal wa blown to steel in afive-ton shaking converter using scrap as cool [,5 ant and burnt lime asslag former. Pure oxygen (99%) was blown through a water cooled lancewith an opening of 30 millimeters.

Input:

4-900 kgs. hot metal 100 kgs. scrap 300 kgs. lime 30 kgs. ferromanganese 314 Nm. oxygen Output:

43 00 kgs. steel 700 kgs. slag Blowing time: 17.9 minutes The course ofthe heat is shown in the table below:

Period N o: 0 l 2 Scrap, lrgs 100 Lime, kgs 300 Oxygen, N1IL3 2 53Blowing time, minutes. 14. 9 3

Sample N o O l 2 Metal temperature, C 1, 140 1, 575 1, 642

Analyses 0 1 2 Metal, percent:

l. 45 0. 36 0. 18 0. 210 0. 025 0. 025 0. 028 O. 018 0. 012 N 0. 005 0.002 0. 003 Slag, percent:

CaO 54. 4 39. 6 MgO--- 2. 25 2.80 M 10.8 9. 24 S102 8. 09 5. 98 P 0 4.56 3.62 Fe (total)- 12. 6 27. 6

Scrap and lime were added before the start of the blowing. A thick,fluid slag was formed at the end of the first period of blowing.

The advantages of this method according to the invention are many. Someof them are enumerated hereinbelow.

The melts are blown without any ejections and very rapidly.

The low temperature procedure demands large cooling agent additionswhich preferably in the form of scrap iron decrease the charge costs andincrease the yield.

With the cold and solid slags the durability of the lining is very good.

The contents of all accessary elements in the iron, sulphur included,are very rapidly decreased to low values due to the good reactivity ofthe solid slags, so that the refining process is ready at any desiredfinal carbon content.

Since one has only to go by the final carbon content the handling of themelt is very simple.

At the low temperatures the dust losses are small.

What is claimed is:

1. Method for refining pig iron to steel which comprises substantiallycontinuously blowing a gaseous oxygen carrier on to a bath of molten pigiron and slag maintained in natural virbation in a melting container ofround cross section by exterior means, said vibration being sufficientto cause intermixing of the surface layers of the bath, maintaining theslag in substantially solid state during the major portion of theblowing period. and until a short time before attaining that carboncontent at which the blowing is to be ended, liquifying said slag andshortly thereafter ending said blowing.

2. Method as defined in claim '1 in which the temperature of the bath ismaintained below the pouring temperature until near the end of theblowing by the addition thereto of a cooling agent.

3. Method as defined in claim [1 in which the pig iron is a highphosphorus pig iron, the melting container has a basic lining, the slagis maintained supersaturated with CaO and near the end of the blowingundissolved slag constituents are caused to be dissolved in the slag byincreasing the temperature of the slag.

4. Method as defined in claim 1 in which the pig iron is a highphosphorus pig iron, the melting container has a basic lining, the slagis maintained supersaturated with Ca() and near the end of the blowingundissolved slag constituents are caused to be dissolved in the slag byincreasing the FeO content of the slag.

5. Method as defined in claim 1 in which the pig iron is a highphosphorus pig iron, the melting container has a basic lining, the slagis maintained supersaturated with C110 and near the end of the blowingundissolved slag constituents are caused to be dissolved in the slag byadding a flux to the slag.

6. Method as defined in claim 1 in which the pig iron is low phosphoruspig iron, the melting container has a basic lining, the slag ismaintained supersaturated with 2CaO-Sio and near the end of the blowingundissolved slag constituents are dissolved therein by increasing thetemperature of the slag.

7. Method as defined in claim 1 in which the pig iron is low phosphoruspig iron, the melting container has a basic lining, the slag ismaintained supersaturated with a CaO-SiO and near the end of the blowingundissolved slag constituents are dissolved therein by increasing theFeO content of the slag.

8. Method as defined in claim 1 in which the pig iron is low phosphoruspig iron, the melting container has a basic lining, the slag ismaintained supersaturated with 2 CaO-Sio and near the end of the blowingundissolved slag constituents are dissolved therein by adding a flux tothe slag.

9. Method as defined in claim 1 in which the pig iron is low in P and S,the melting container has .an acid lining, the slag is maintainedsupersaturated with S10 and near the end of the blowing undissolved slagconstituents are dissolved therein by increasing the temperature of theslag.

10. Method as defined in claim 1 in which the pig iron is low in P andS, the melting container has an acid lining, the slag is maintainedsupersaturated with S10 and near the end of the blowing undissolved slagconstituents are dissolved therein by increasing the FeO content of theslag.

11. Method as defined in claim 1 in which the pig iron is low in P andS, the melting container has an acid lining, the slag is maintainedsupersaturated with S10 and near the end of the blowing undissolved slagconstituents are dissolved therein by adding a flux to the slag.

References Cited by the Examiner UNITED STATES PATENTS 879,480 2/ 1908Von Maltitz -52 2,258,850 10/1941 Eu-leustein et al. 7552 2,668,7592/1954 Tenenbaum 7552 2,671,018 3/1954 Graet' 75--52 2,741,554 4/1956'Rinesoh 7560 2,826,488 3/1958 Bowden 7552 2,893,861 7/1959 Rinesch 7552FOREIGN PATENTS 723,615 2/ 1955 Great Britain.

806,776 12/1958 Great Britain.

BENJAMIN HENKIN, Primary Examiner.

DAVID L. RECK, Examiner.

1. METHOD FOR REFINING PIG IRON TTO STEEL WHICH COMPRISES SUBSTANTIALLYCONTINUOUSLY BLOWING A GASEOUS OXYGEN CARRIER ON TO A BATH OF MOLTEN PIGIRON AND SLAG MAINTAINED IN NATURAL VIBRATION IN A MELTING CONTAINER OFROUND CROSS SECTION BY EXTERIOR MEANS, SAID VIBRATION BEING SUFFICIENTTO CAUSE INTERMIXING OF THE SURFACE LAYERS OF THE BATH, MAINTANING THESLAG IN SUBSTAITNALLY SOLID STATE DURING THE MAJOR PORTION OF THEBLOWING PERIOD AND UNTIL A SHORT TIME BEFORE ATTAINING THAT CARBONCONTENT AT WHICH THE BLOWING IS TO BE ENDED, LIQUIFYING SAID SLAG ANDSHORTLY THREAFTER ENDING SAID BLOWING.